The present invention relates generally to a hybrid field-sequential display (HFSD) for video image simulation apparatus.
The development of miniature color displays that are suitable for helmet-mounted display (HMD) applications is very difficult. For monochrome HMD applications, the main approach has been to miniaturize the conventional monochrome display technology (i.e., the monochrome CRT), but this will not work for full-color applications. Conventional color-display technology is based on the shadowmask CRT, and this technology cannot be suitably miniaturized. Therefore, new approaches must be developed.
One solution is to use three monochrome displays, each of which displays an image in one of the three primary colors (i.e., red, green, and blue), and superimpose the images. This yields a full-color display having the same resolution as the component monochrome displays. However, the resulting device is bulky.
U.S. Pat. No. 4,427,977 to Carollo et al provides a teaching of a helmet mounted display simulator including a plurality of image generators equal in number to a plurality of controlled cathode ray tubes, each of which generates a monochrome image (red, green, and blue) to be transmitted and displayed to the viewer. The several monochrome images are combined by a set of dichroic prisms and transmitted by fiber optics.
Another approach is to build a miniature solid-state display, such as an LCD having a color-filter mosaic. Displays of this type are currently being used in commercial miniature television sets. The problem with this technology is that several pixels are required to form a full-color pixel, which makes it difficult to build a mosaic-filter LCD having a sufficient number of pixels for HMD applications. For example, a state-of-the-art color LCD having 1024.times.1024 pixels in a quad configuration provides a resolution of only 512.times.512 full-color pixels. Furthermore, if its image is presented in a 60-degree field-of-view (such as might be used for a wide field-of-view HMD), each pixel will subtend 3.5 arc-minutes of visual angle. Therefore, the individual pixels will be discernible and no spatial color mixture will occur in the observer's visual system. The need to miniaturize the HMD further complicates matters because this leads to very high pixel-density requirements.
Another approach is to use temporal integration in the observer's visual system to achieve full color. This technique is known as field-sequential presentation and involves presenting superimposed but temporally separate red, green, and blue images in rapid succession so that the visual system fuses them into a single, non-flickering, full-color image. The most common example of a full-color FSD is the single-gun, three-primary, penetration-tube CRT. The main problem with this technology is limited color gamut. Also, luminous efficiency suffers, relative to a two-primary penetration tube, because of the need for an additional phosphor-barrier layer. (It should be noted here that three-gun penetration tubes, which are not FSDs, can be built. However, it is not possible to fit three electron guns in a miniature package.)
A much older full-color FSD technology involves placing a rotating filter wheel in front of a white CRT and synchronizing image generation on the CRT with the filter's movement. It is impractical, of course, to place a rotating filter wheel on a helmet. The system can be installed elsewhere, though, and the image routed to the helmet via a fiber-optics cable. The main disadvantage is that resolution is then limited by the cable and will deteriorate as individual fibers break. If the helmet is to be removable from the cockpit, there are also difficulties associated with reconnecting the cable properly. Still another problem is that light-loss in the cable reduces image luminance, which is already diminished by the filters. In principle, this problem can be overcome by using a transmissive LCD (or other solid-state display) to generate the image, rather than a CRT. This is because an LCD's luminance can be increased by increasing its backlighting, whereas a CRT's luminance is ultimately limited by phosphor efficiency, beam current limitations, and beam-current vs. spot-size tradeoffs. The problem here is that, to achieve a 60-Hz frame rate, the color fields must be presented at 180 Hz, and no contemporary LCD (or other solid-state display technology) can support this refresh rate.
A more modern and compact version of the filter-wheel idea utilizes a liquid-crystal shutter. The shutter is essentially a spectrally selective polarizing liquid-crystal filter. Rotating the liquid-crystal to one orientation allows light from one portion of the spectrum (e.g. green) to pass and rotating 90 degrees allows light from another portion of the spectrum (e.g., red) to pass. The rotation can be very rapid and commercial two-primary displays which support a 60-Hz frame rate (120-Hz color-field rate) are currently being marketed. The shutter is also compact enough for helmet-mounted applications. In principle, a full-color system can be produced by sandwiching two shutters (e.g., a red/cyan plus a yellow/blue) together and increasing the color-field rate to 180 Hz, but this has not been demonstrated. (The basic problem is that the CRT's yoke must be cooled to run at such speed.) Another drawback is that the shutter wastes even more light than a rotating filter wheel because of light loss from the polarizers.